In the field of computer graphics, the rendering of two-dimensional objects is of fundamental importance. Two-dimensional objects, such as character shapes, corporate logos, and elements of an illustration contained in a document, are rendered as static images or as a sequence of frames comprising an animation. There are numerous representations for two-dimensional objects and it is often the case that one representation is better than another representation for specific operations such as rendering and editing. In these cases, a conversion from one form to another is performed.
Although we focus here on digital type, possibly the most common and important two-dimensional object, the following discussion applies to all types of two-dimensional objects.
We begin with some basic background on digital type. A typical Latin font family, such as Times New Roman or Arial, includes a set of fonts, e.g., regular, italic, bold and bold italic. Each font includes a set of individual character shapes called glyphs. Each glyph is distinguished by its various design features, such as underlying geometry, stroke thickness, serifs, joinery, placement and number of contours, ratio of thin-to-thick strokes, and size.
There are a number of ways to represent fonts, including bitmaps, outlines, e.g., Type 1 [Adobe Systems, Inc. 1990] and TrueType [Apple Computer, Inc. 1990], and procedural fonts, e.g., Knuth's Metafont, with outlines being predominant. Outline-based representations have been adopted and popularized by Bitstream Inc. of Cambridge, Mass., Adobe Systems, Inc. of Mountain View, Calif., Apple Computer, Inc., of Cupertino, Calif., Microsoft Corporation of Bellevue, Wash., URW of Hamburg, Germany, and Agfa Compugraphic of Wilmington, Mass.
Hersch, “Visual and Technical Aspects of Type,” Cambridge Uneversity Press. 1993 and Knuth, “TEX and METAFONT: New Directions in Typesetting,” Digital Press, Bedford, Mass. 1979, contain comprehensive reviews of the history and science of fonts.
Of particular importance are two classes of type size: body type size and display type size. Fonts in body type are rendered at relatively small point sizes, e.g., 14 pt. or less, and are used in the body of a document, as in this paragraph. Body type requires high quality rendering for legibility and reading comfort. The size, typeface, and baseline orientation of body type rarely change within a single document.
Fonts in display type are rendered at relatively large point sizes, e.g., 36 pt. or higher, and are used for titles, headlines, and in design and advertising to set a mood or to focus attention. In contrast to body type, the emphasis in display type is on esthetics, where the lack of spatial and temporal aliasing is important, rather than legibility, where contrast may be more important than antialiasing. It is crucial that a framework for representing and rendering type handles both of these classes with conflicting requirements well.
Type can be rendered to an output device, e.g., printer or display, as bi-level, grayscale, or colored. Some rendering engines use bi-level rendering for very small type sizes to achieve better contrast. However, well-hinted grayscale fonts can be just as legible.
Hints are a set of rules or procedures stored with each glyph to specify how an outline of the glyph should be modified during rendering to preserve features such as symmetry, stroke weight, and a uniform appearance across all the glyphs in a typeface.
While there have been attempts to design automated and semi-automated hinting systems, the hinting process remains a major bottleneck in the design of new fonts and in the tuning of existing fonts for low-resolution display devices. In addition, the complexity of interpreting hinting rules precludes the use of hardware for font rendering. The lack of hardware support forces compromises to be made during software rasterization, such as the use of fewer samples per pixel and poor filtering methods, particularly when animating type in real time.
Grayscale font rendering typically involves some form of antialiasing. Antialiasing is a process that smoothes out jagged edges or staircase effects that appear in bi-level fonts. Although many font rendering engines are proprietary, most use supersampling, after grid fitting and hinting, with 4 or 16 samples per pixel followed by down-sampling with a 2×2 or 4×4 box filter, respectively.
Rudimentary filtering, such as box filtering, is justified by the need for rendering speed. However, even that approach is often too slow for real-time rendering, as required for animated type, and the rendered glyphs suffer from spatial and temporal aliasing.
Three important trends in typography reveal some inherent limitations of prior art font representations and associated methods and thus provide the need for change.
The first trend is the increasing emphasis of reading text on-screen due to the dominant role of computers in the office, the rise in popularity of Internet browsing at home, and the proliferation of PDAs and other hand-held electronic devices. These displays typically have a resolution of 72–150 dots per inch, which is significantly lower than the resolution of printing devices.
This low-resolution mandates special treatment when rasterizing type to ensure reading comfort and legibility, as evidenced by the resources that companies such as Microsoft and Bitstream have invested in their respective ClearType and Font Fusion technologies.
The second trend is the use of animated type, or kinetic typography. Animated type is used to convey emotion, to add interest, and to visually attract the reader's attention. The importance of animated type is demonstrated by its wide use in television and Internet advertising.
The third trend is the proliferation of display devices that incorporate numerous layouts for components of pixels of such displays. Vertically and horizontally striped RGB components have been the standard arrangement for conventional displays, as described in U.S. Pat. No. 6,188,385 “Method and apparatus for displaying images such as text”, Hill et al. Arranging the components differently, however, has numerous advantages, as described in U.S. Patent Application publication number 20030085906 “Methods and systems for sub-pixel rendering using adaptive filtering”, Elliott et al.
Unfortunately, traditional outline-based fonts and corresponding methods have limitations in all of these areas. Rendering type on a low-resolution display requires careful treatment in order to balance the needs of good contrast for legibility, and reduced spatial and/or temporal aliasing for reading comfort.
As stated above, outline-based fonts are typically hinted to provide instructions to the rendering engine for optimal appearance. Font hinting is labor intensive and expensive. For example, developing a well-hinted typeface for Japanese or Chinese fonts, which can have more than ten thousand glyphs, can take years. Because the focus of hinting is on improving the rendering quality of body type, the hints tend to be ineffective for type placed along arbitrary paths and for animated type.
Although high quality filtering can be used to antialias grayscale type in static documents that have a limited number of font sizes and typefaces, the use of filtering in animated type is typically limited by real-time rendering requirements.
Prior art sub-pixel rendering methods, like those described in U.S. Pat. No. 6,188,385, have numerous disadvantages pertaining to all three trends.
First, they require many samples per pixel component to get adequate quality, which is inefficient. When rendering on alternative pixel layouts comprising many components, e.g., such as the layouts described in U.S. patent application publication number 20030085906, their methods become impractical. Second, they exploit the vertical or horizontal striping of a display to enable reuse of samples for neighboring pixel components, which fails to work with many alternative pixel component layouts. Third, they use a poor filter when sampling each component because of the inefficiencies of their methods when using a better filter.
Fourth, the methods taught do not provide any measure for mitigating color fringing on alternative pixel component layouts. Fifth, translations of a glyph by non-integer pixel intervals require re-rendering of the glyph. Re-rendering usually requires re-interpreting hints, which is inefficient. Sixth, hints are often specific to a particular pixel component layout, and therefore must be redone to handle the proliferation of alternative pixel component layouts. Redoing hints is both expensive and time consuming.
Rendering Overlapping Objects
When two or more objects are rendered, their rendered images may overlap. For example, the antialiased edges of two glyphs in a line of text may overlap when the glyphs are placed close together. As another example, a single Kanji glyph may be represented by a composition of several elements, such as strokes, radicals, or stroke-based radicals, which may overlap when they are combined to render the single Kanji glyph.
In such cases, a rendering method must handle a region where the objects overlap. There are several methods in the prior art for handling such overlap regions. The “Painter's Algorithm” is a common approach used in computer graphics for two-dimensional and three-dimensional rendering. In the Painter's Algorithm, objects are ordered back-to-front and then rendered in that order. Pixels determined by each rendering simply overwrite corresponding pixels in previous renderings.
Other prior art methods blend color or intensity values of overlapping pixels, i.e., those methods combine the color or intensity values according to a rule, such as choosing a maximum or a minimum value or performing an arithmetic average of the overlapping pixels. Some of those methods use alpha values associated with each pixel to blend the values of the overlapping pixels using a technique called alpha blending.
Those prior art methods can be problematic for a number of reasons.
For example, the Painter's Algorithm results in color artifacts between closely spaced glyphs when rendering on liquid crystal displays (LCDs), organic light emitting diodes (OLEDs), or similar display technologies with separately addressable pixel components.
Prior art methods that blend pixel colors or intensities require additional computation and storage for alpha values and exhibit various artifacts such as edge blurring or edge dropout depending on the blending method used.
In addition, coverage values determined for a set of overlapping objects using prior art coverage-based antialiasing cannot, in general, be blended together to represent the actual coverage of the combined object.
Another prior art approach for handling overlapping objects combines the objects to produce a composite object prior to rendering. For example, for an outline-based glyph composed of multiple elements, the outlines of the elements are joined to form a single outline description prior to rendering. Similarly, for rendering a stroke-based glyph composed of multiple strokes, the strokes are combined into a single set of strokes before rendering.
For object elements represented as distance fields, the distance fields can be combined into a single distance field prior to rendering using CSG operations as described by Perry et al., “Kizamu: A System for Sculpting Digital Characters,” Proceedings ACM SIGGRAPH 2001, pp. 47–56, 2001. When the composite object is represented as an adaptively sampled distance field, the composite object can require significantly more storage than the total storage required by the elements because the combining may introduce fine detail such as very thin sections or corners into the composite object that are not present in any element.
All of those prior art methods that combine prior to rendering require additional storage space and complex operations to generate the composite object. Furthermore, those methods require two passes, one to build the composite object and one to render the composite object.
Generating and Rendering Component-Based Glyphs
An Asian font, such as a Chinese, Japanese, or Korean font, can include 10,000 or more glyphs. In order to reduce memory requirements, glyphs in such fonts can be represented as compositions of a common set of components, herein referred to as elements, such as strokes or radicals. These common elements are then stored in a memory as a font library and combined either prior to rendering or during rendering to produce a composite glyph.
Prior art methods define the elements using outline descriptors, typically Bezier curves, or stroked skeletons. The elements can be combined prior to rendering into a single shape descriptor, such as a combined outline or a combined set of strokes. Alternatively, each element can be rendered independently, producing, for each pixel, either antialiased intensities or coverage values from the elements that are combined to produce a final antialiased intensity or coverage value for the pixel. Both approaches have problems as described above.